jet grouting_strenght stiffness parameters

A.E.C. van der Stoel 1/7

STRENGTH & STIFFNESS PARAMETERS OF JET GROUTING COLUMNS : FULL SCALE TEST AMSTERDAM Almer E.C. van der Stoel1, Harro J. van Ree2 ABSTRACT : Using different jet grouting methods and variable diameters, a series of samples have been taken by means of core drilling of jet grouting columns made in soft stratified soil. Average values of the compressive strength, tensile strength and secant modulus are presented as are the jet grouting process pa-rameters and soil conditions. The influence of the water-cement ratio and the column diameter is discussed. Finally, four (exponential) expressions are presented interrelating the compressive strength and the tensile strength and the compressive strength and the secant modulus for jet grouted sand and jet grouted clay. INTRODUCTION General Information

The North/South metroline Amsterdam will use the shield tunnelling method for construction of two ∅ 6,5 m metro tunnels. Therefore, due to settlements, complications with some historical buildings founded on pile foundations may occur. To prevent these complications, mitigating measures by means of stabilising the soil using injection-techniques, are planned at these locations. The lack of experience with injection in the vicinity of pile foundations, in combination with the fact that the non homogeneous, soft, stratified soil in Amsterdam has been injected to a limited degree, has led to the development of a “Full Scale Injection Test”. Because of national interest, the Full Scale Injection Test is a co-operation of North/South Metroline Con-sulting engineers (75%), Centre for Underground Construction Studies (20%) and Delft University of Tech-nology (5%). More information concerning the project can be found in other North/South metroline papers in ITA congress proceedings 1997, 1998 and 1999 (Van der Stoel, 1998 & 1999).

Scope of the Paper

The test program is divided into three stages: − stage 1: the testing of several different geophysical surveys. − stage 2: injection of different Amsterdam soil types by means of permeation grouting; − stage 3: injection near pile foundations using (a) permeation-, (b) jet- and (c) compensation grouting. Stage 1 was completed in February 1998 and stage 2 was completed in February 1999. Execution of Stage 3 has finished in December 1999, and the complete first order evaluation results will probably be available in April 2000. This paper outlines the results from laboratory test which were part of the jet grouting (stage 3b). Aim of Stage 3b

The aim of stage 3b is to gain (additional) knowledge concerning jet grouting, with regards to: - strength and stiffness parameters, - influence on surroundings, both soil as well as pile foundations (movement and stress) and - the possible verification methods for determining the column diameter (V.d.Stoel & V.d.Bliek,2000).

This paper focuses on the first aspect. TEST SET-UP General Set-Up

1 Almer E.C. van der Stoel, Delft University of Technology Faculty of Civil Engineering & Geosciences and Fac-

ulty of Architecture & North/South Metroline, Entrada 231, 1096 EG, Amsterdam, Netherlands 2 Harro J. van Ree, North/South Metroline, Entrada 231, 1096 EG, Amsterdam, Netherlands

The main principle of the test is to load a series of wooden and concrete piles using jacks, and measuring the loads, movements, water-pressures and stresses during grouting. For the loading 8750 kN of ballast, consisting of a steel frame and concrete & sand dead weight was used (Van der Stoel, 2000). Soil Conditions

A detailed description of the soil conditions is given in Van der Stoel (1998). A brief summary is pre-sented here (with NAP = Amsterdam Ordnance Datum, the Dutch reference level for vertical measurements): • street level at NAP + 0,5 m; groundwater level at NAP –0,8 m; • NAP + 0,5 m to NAP –13 m: Holocene sequence consisting respectively of

– Sand and rubble (made ground): NAP +0,5m to NAP –0,5m; – Layer 1: Clay/Peat NAP –0,5m to NAP-5,5m; – Layer 2: Upper clay layer NAP –5,5 m to NAP –8,0 m; – Layer 3: Lower clay layer (silty/sandy claylayer) NAP –8,0m to NAP –12,5m;

• Layer 4: NAP –12,5 m to NAP –15 m; 1st sand layer; cone resistance (CPT) 6 - 30 MN/m²; • Layer 5:NAP –15 m to NAP –18 m; Intermediate layer; cone resistance 2 -12 MN/m²; • Layer 6 NAP –18 m to NAP –30 m; 2nd sand layer; dense to very dense; cone resistance 15 - 45 MN/m²; • Layer 8 NAP –30 m to NAP –45 m; Eem clay layer; cone resistance 2 - 3 MN/m².

The piezometric surface of the 1st and 2nd sand layer is NAP –1,4 m. Process Parameters Jet Grouting

Jet grouting is an eroding process, and therefore both displacement as well as relaxation of the soil can occur during grouting. For the creation of the jet grouting columns, a one-and two phase jet grouting system (single and double jet) combined with pre-cutting has been used.

A single jet system consists of jetting/eroding a water-cement mixture with the soil. In a double jet sys-tem the water cement mixture is surrounded by an air jet, making the jetting more powerful and creating a larger diameter column. Six different grout columns have been made, as shown in Figure 1 and Table 1.

For all jet grouting columns, precutting by means of water-jetting was used for the cohesive soil-layers (layer # 2, 3 and 8). The aim of the precutting is to realise a column with sufficient diameter and to increase the strength and stiffness of the grout. Process parameters of the jet grouting are displayed in Table 1. The rotation speed of the nozzles is the same for all columns: 6-8 r/min.

Table 1 : Process parameters used for the jetting of the columns. Column ∅

(m)

Grouting

From To

NAP (m)

wcr Jet

system

Lift speed

precutting / grouting

(sec/2cm)

Nozzle

diameter

(mm)

Precutting

pressure

(bar)

Flow Rate

(l/min)

Grouting

pressure

(bar)

A 1 -35 -2 1.0 single 4 / 6 4.0 200 156 400

B 1 -35 -2 1.0 single 4 / 6 4.0 200 156 400

C 2 -35 -2 1.0 double 8 / 12 5.0 400 244 400

D 2 -35 -2 1.0 single 8 / 12 5.0 400 257 450

X1 1 -22 -2 0.8 single 4 / 6 4.0 200 152 400

X2 1 -22 -2 1.2 single 4 / 6 4.0 200 158 400

TEST RESULTS Excavation

Figure 1 shows the excavation of the jet grouting site (left, X1 not shown) and the designed columns (right, X1 and X2 not shown). In between columns B and C some soil is still visible. All the columns have been inspected and the average diameter has been measured. It could be immediately recognised that: • the columns roughly all have a constant diameter; • column C has a somewhat square shape (which could not be explained yet); • the average diameter of columns A (∅ 1.45 m), B (∅ 1.40 m), C (∅ 2.60 m) and X2 (1.40m) exceed the

intended diameter, the diameter of column D (∅ 1.55 m) is smaller then the intended diameter. Design Considerations


When determining the strength and stiffness of the grout, some design considerations (influences on strength and stiffness of the grout) have to be taken into account, like the air used with column C and the wa-ter cement ratio (wcr) of columns X1 and X2 which is respectively 0.8 and 1.2.

Figure 1 : excavated site (left) column design (right)

For interpretation purposes the laboratory tests have been divided in five categories, as shown in Table 2. Core Drilling and Sampling Program

To test the quality of the grout, core drilling of column A, B, C, D, X1 and X2 has been executed over the full length of the columns. The core drilling has been executed for the designed ∅ 1m and ∅ 2m columns at respectively 0,20m and 0,40m distance from the centre of the jet grout column, considering that centric drill-ing of the jet grouting columns gives no representative samples. Both the verticality of the jet grouting col-umn as well as of the core drilling were checked using inclinometer measurements (maximum deviation of 0,35 m ≈ 1 %). A total of 255 samples has been taken from drilled cores (Table 2). Compressive Strength and Stiffness of the Grout

The stiffness (Ecm) and compressive strength (fc) were determined using Unconfined Compressive Strength (UCS) tests, in accordance with the DIN 18136. The secant modulus of the stiffness is determined by registering the strain and the force at 30%-70% of the failure force on the sample. The tensile strength (fct,sp) was determined using Brazilian Spilt (BS) tests. The results of the tests are presented in Table 2.

When studying the table some conclusions can be drawn: a) there is a wide scatter in the data / large standard deviations; b) the use of the double jet system (category 4) has a positive influence on the diameter of the column, the

strength parameters are however significantly lowered; c) the strengths for category 2 (wcr=1) are not intermediate between category 1 and 3 (wcr =0.8 and 1.2).

The reason for a) and c) can probably be found in the fact that the number of samples that were taken for categories 1, 3 and 4 are significantly lower then the number of samples taken for category 2. The excellent results for layer 6 in category 3 for instance, are based upon only two samples taken form one core.

The reason for b) is rather obvious and has to do with air being enclosed in the jet grouting column during the jetting.

Correlation

To be able to use strength and stiffness parameters of jet grouted material in the design process, it is ex-tremely useful to derive formulas that interrelate these parameters. Using the test results two typical rela-tions are presented here. A distinction has been made between the clay (#2, 3 & 8) and the sand (#4, 5 & 6) layers. The relations for a water-cement ratio of 1,0 (wcr=1.0).

Compressive Strength fc- Tensile Strength fct,sp

Figure 2 and Figure 3 display the test results for the clay and sand layers respectively. It should be noted that more UCS-tests then BS-tests were taken, so sometimes multiple fc are compared with a single fct. For concrete, a number of empirical formulae connecting fc and fct,sp have been suggested, many of them of the type: fct,sp = k (fc)

n (Neville, 1997). Table 2 : Average (x ) and standard deviation (σ) of strength and stiffness parameters of the samples

Category 1 Category 2 Category 3 Category 4 Category 5

column(s) X1 A, B X2 C A, B & D

diameter ∅ (m) 1.5 1.5 1.5 2.5 1.5

system Single Single Single Double Single

wcr 0.8 1.0 1.2 1.0 1.0

Laye

r

Parameter

Nu

mb

er o

f sam

ple

s

x

σ x

σ x

σ x

σ x

σ

fc (MPa)

2 Upper clay layer 24 5.4 1.0 3,4 1,0 3.0 0.7 - - 3.3 1.0

3 Lower clay layer 26 13.9 10.2 11,8 8,9 3.4 2.5 3.3 0.4 9.9 8.8

4 1st sand layer 27 7.9 3.6 19,6 12,6 4.4 3.0 2.9 0.9 17.1 12.4

5 In-between layer 21 11.1 1.9 12,9 8,3 8.6 1.7 7.3 1.0 15.4 9.0

6 2nd sand layer 43 23.0 16.4 20,3 11,6 33.1 13.2 8.5 2.2 18.7 11.2

8 Eemclay layer 21 - - 14,7 7,4 - - 5.6 1.1 12.6 6.7

Ecm (MPa)

2 Upper clay layer 24 2,050 702 1197 914 982 473 - - 1,096 837

3 Lower clay layer 26 3,391 1,839 2623 1166 1,194 886 933 230 2,335 1,193

4 1st sand layer 27 2,830 934 3449 1249 1,330 746 924 335 3,427 1,164

5 In-between layer 21 3,412 971 2551 1007 2,264 399 1,737 399 2,913 991

6 2nd sand layer 43 3,564 1,622 3337 1215 5,222 1,418 2,334 551 3,268 1,140

8 Eemclay layer 21 - - 2687 673 - - 1,532 427 2,775 632

fct,,sp (MPa)

2 Upper clay layer 18 1.0 0.1 0,6 0,7 0.5 0.1 - - 0.6 0.6

3 Lower clay layer 11 1.8 0.8 0,7 0,4 1.0 0.5 0.6 0.0 0.6 0.4

4 1st sand layer 22 1.0 0.1 2,0 0,1 0.7 0.3 - 0.5 2.0 0.2

5 In-between layer 11 1.5 0.1 1,5 0,8 1.2 0.2 1.1 0.0 1.4 0.8

6 2nd sand layer 23 2.0 2.0 1,9 1,0 3.2 0.7 1.0 0.3 1.6 0.9

8 Eemclay layer 8 - 0.2 0,7 - - - 0.9 0.2 0.8 0.2


Figure 2 : tensile strength fct,sp vs. compressive strength fcfor jet grouted sand layers

Figure 3 : tensile strength fct,sp vs. compressive strength fc for jet grouted clay layers

0 ,0

0 ,2

0 ,4

0 ,6

0 ,8

1 ,0

1 ,2

1 ,4

1 ,6

1 ,8

2 ,0

0 5 1 0 1 5 2 0 2 5 3 0

c o m p r e s s iv e s t r e n g th f c ( M p a )

tens

ile s

tren

gth

f ct,s

p (

Mpa

)

J e t g r o u te d c la y w c r = 1 ,0



3,0

, 4,0cs pc t

ff ⋅=

0

1.000

2.000

3.000

4.000

5.000

6.000

7.000

0 5 10 15 20 25 30 35 40 45 50

com press ive s trength f c (M P a)

Sec

ant m

odul

us

Ecm

(M

Pa)

je t g rou ted sand wcr=1,0



cc m fE ⋅= 8 0 0

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

0 5 10 15 20 25 30 35 40 45

com pressive s tren g th f c (M pa)

ten

sile

str

en

gth

f c

t,s

p (M

pa

Jet grouted sand wcr=1,0



6,0, 3,0 cspct ff ⋅=

Figure 4 : secant modulus Ecm vs. compressive strength fc for jet grouted sand layers

Figure 5 : secant modulus Ecm vs. compressive strength fc for jet grouted clay layers Because of the similarities between jet grouted material and concrete this relation was applied to the test data, resulting in expressions (1) and (2):

jet grouted sand: fct,sp = 0,3 (fc)3/5 (1)

jet grouted clay: fct,sp = 0,4 (fc)

3/10 (2) The results for the tests with the higher and lower wcr are also displayed in the figures. Because the number of tests was much lower, it is only stated here that a higher wcr shows a tendency of shifting the line a little higher and a lower wcr shows a tendency of shifting the line a little lower. Compressive Strength fc – Secant Modulus Ecm

In Figure 4 and Figure 5 show the test results for the clay and sand layers are shown respectively. Here also an empirical formula of the type: Ecm = k (fc)

n is used, resulting in expressions (3) and (4): jet grouted sand: Ecm = 800 (fc)

1/2 (3) jet grouted clay: Ecm = 500 (fc)

2/3 (4)

The results for the tests with the higher and lower wcr are also displayed in the figures. It is remarkable that a lower wcr doesn’t very much influence the relation, a higher wcr however does result in a higher se-cant modulus using a comparable compressive strength.

Because the number of tests with a high compressive strength is significantly lower then the number of tests with a high compressive strength, the expressions for Ecm and fct,sp are more sensitive for variation with higher values of fc. CONCLUSIONS & RECOMMENDATIONS

The presented test results show that a higher or lower wcr significantly influences the strength and stiff-

ness of the grouted clay layers. A wide scatter in the strength and stiffness parameters was found for the dif-ferent layers and rather big standard deviations were found per layer. The use of the double jet system has a positive influence on the diameter of the column, the strength parameters are however significantly lowered. The use of precutting in the clay layers resulted in more than satisfactory strength and stiffness parameters.

For both sand as well as clay an exponential relation between the tensile strength and the compressive strength by the type: fct,sp = k (fc)

n could be derived. There is a clear difference between sand and clay. For

0

1 .0 0 0

2 .0 0 0

3 .0 0 0

4 .0 0 0

5 .0 0 0

6 .0 0 0

0 5 1 0 1 5 2 0 2 5 3 0 3 5

c o m p re s s iv e s tre n g th f c (M P a )

Sec

ant m

odul

us

Ecm

(M

Pa)

je t g ro u te d c la y w c r= 1 ,0



3 25 0 0 cc m fE ⋅=


the relation between the secant modulus and the compressive strength also an exponential relation was found: Ecm = k (fc)

n. Here the difference between clay and sand was however statistically less significant. It is emphasised that because of the wide scatter that is (generally) found when analysing jet grouting

data, sufficient risk assessment has to be conducted. Depending on the project and the function of the jet grouting body this means that “safety factors” of 2 to 3 are strongly recommended. REFERENCES Neville, A.M., 1997, Properties of concrete, p.306, Essex Addison Wesley Longman Ltd. Stoel, A.E.C. van der, 1998. “Soil Grouting: Full Scale Injection test North/South metro line Amsterdam”,

Tunnels and Metropolises, Proceedings of the World Tunnel Congress 1998 on Tunnels and Metropolises São Paolo Brazil, Rotterdam: Balkema.

Stoel, A.E.C. van der; A.F. van Tol, 1999, “Full Scale Injection Amsterdam: Results stage 1 and 2”, Pro-ceedings of the ITA World Tunnel Congress 1999 Oslo Norway, Rotterdam: Balkema.

Stoel, A.E.C. van der, 2000,“Injection / grouting near pile foundations: Full Scale Test Amsterdam”, Geo-technical Aspects of Underground Construction in Soft Ground, IS’99, Tokyo, Japan, Rotterdam, Balkema

Stoel, A.E.C. van der & M.P.A. van den Bliek, 2000, “Verifying the diameter of jet grouting columns: Full Scale Test Amsterdam”, 4th GIGS, Helsinki, Finland, Rotterdam, Balkema

jet grouting_strenght stiffness parameters

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